1. Technical Field
This disclosure relates to voltage-to-current sensing circuits and more particularly to an architecture of voltage-to-current sensing circuit with a reduced number of high voltage components, and to a multi-phase DC-DC converter having a plurality of voltage-to-current sensing circuits sharing a common bias terminal.
2. Description of the Related Art
In single-phase or multi-phase DC-DC switching voltage converters the current flowing throughout each phase winding is sensed in order to properly drive the converter. The current flowing throughout the phase windings is sensed by inserting a sense resistance Rsense in series with the winding (FIG. 1a), or exploiting the DC resistance of the winding, or by coupling the phase winding with a R-C line (FIG. 1b), wherein the sense capacitor Cdcr withstands the DC component of the voltage drop on the phase winding, then by sensing the voltage drop on the sensing terminals CS+, CS−.
A commonly used current sensor is shown in FIG. 2. It substantially comprises an operational amplifier opamp0 that controls an output transistor Mo such to make the voltage on the terminal COMP substantially equal to the voltage on the terminal CS+ by forcing a sense current Iout throughout the comparison resistor Rcomp. In steady-state conditions, the sense current Iout is:
                              I          out                =                              I            coil                    ·                                    R              sense                                      R              comp                                                          (        1        )            and may be easily mirrored and distributed to circuits that drive the phase winding. The comparison resistor Rcomp may be a discrete component installed on a printed circuit board or be integrated in the chip that contains the DC-DC voltage converter.
A main drawback of this type of sensing circuit consists in that the regulated voltage Vout, output by the DC-DC converter, may be relatively high, that is it may exceed the maximum voltage that common low voltage components may safely withstand. Therefore, in order to make the above prior circuit capable of sensing a current flowing in a line at a relatively high voltage, the transistor Mo and thus the whole operational amplifier OPAMP0 is made of high voltage components. The same conclusions would be attained if a matched R-C line as shown in FIG. 1b was used instead of a sense resistor Rsense connected in series with the phase winding.
High voltage components are area consuming and due to large parasitics they are inherently slower than the low voltage components and require large driving currents. For these reasons it is desirable to reduce the number of high voltage components as much as possible in the design of voltage-to-current sensing circuits.
Other prior voltage-to-current sensing circuits are disclosed in U.S. Pat. No. 8,203,323 in the name of Texas Instruments Inc. and in the article by S. Herzer et al. “Capacitive-coupled current sensing and auto-ranging slope compensation for current mode SMPS with wide supply and frequency range”, Proceedings of ESSCIRC 2009, 14-18 Sep. 2009, pages 140-143, and are depicted in FIGS. 3 and 4, respectively. Both these prior circuits utilize high voltage switched capacitors, that are area consuming and slow down the transients of the prior voltage-to-current sensing circuits.
A high-level scheme of another prior voltage-to-current sensing circuit is shown in FIG. 5. The difference current between the current Icomp, flowing throughout the comparison resistor Rcomp, and the bias current Ibias is forced throughout the transistor M3. The output transistor M2 is matched with the transistor M3, is referred to ground as the transistor M3 and is controlled by the same control voltage, thus the output current Iout is substantially a replica of the current Icomp−Ibias.
This circuit draws a relevant comparison current Icomp from the phase line and cannot be used when the current flowing through the phase winding L is sensed using a sense capacitor Cdcr connected as shown in FIG. 1b. 